1. Field of the Invention
The present invention relates to an alkali development-type solder resist useful in production of printed wiring boards, the cured product thereof and the printed wiring board prepared by using the same, and in particular, to an alkali development-type solder resist hardenable by a laser-emitting light source at a maximum wavelength of 350 to 375 nm or 400 to 410 nm, and the cured product thereof, and the printed wiring board obtained by using the same.
2. Description of the Related Art
Printed wiring boards for electronic devices have a solder resist film formed as the outermost layer. The solder resist film is a protective coating material covering the surface of a printed wiring board and preventing adhesion of undesired solder on the circuit surface during application of solder and mounting of components. It is the protective film that protects the copper foil circuit of a printed wiring board from humidity, dust, etc. as a permanent protective mask and also the circuit from electrical problems as an insulator, and is superior in chemical and heat resistance and also resistant to the high temperatures encountered during soldering or gold plating. The solder resist film is generally formed by a photolithographic method of irradiating a patterned high-energy ray through a mask pattern. It is possible to select regions demanding no soldering by using the mask pattern.
A laser direct imaging method of using a laser beam as beam source was commercialized recently as an environmentally friendly photolithographic method for resources and energy conservation. Direct imaging apparatuses are apparatuses drawing a pattern-data image directly on a printed circuit board carrying a laser beam-sensitive photocurable resin composition formed thereon, by direct high-speed laser beam irradiation. It is characteristic in that it demands no mask pattern, allows shortening of the production process and drastic reduction of the cost, and is suitable for multi-kind/small-lot and short-delivery-time production.
In such a direct imaging apparatus, in which all the surfaces in the light-exposing regions are not exposed to light simultaneously as by conventional mask pattern exposure, the light-exposing and non-exposing regions are chosen before exposure of the film, consecutively by on/off of the laser shutter. Thus, it is necessary to expose the film at high speed, to obtain an exposure time close to that obtained by the conventional mask pattern exposure. In addition, the beam sources used by conventional mask pattern exposure were those emitting a light having a wide wavelength range of 300 to 500 nm, such as metal halide lamp, but a semiconductor laser is used instead as the beam source in the direct imaging apparatus, although the beam source and the wavelength may vary according to the application of the photocurable resin composition used, and the wavelength commonly used is 355 nm or 405 nm.
However, conventional solder resists, when exposed to a light at a wavelength of 355 nm or 405 nm in a direct imaging apparatus, do not form films satisfying the requirements, such as in heat resistance and insulating property, demanded for solder resists. This is because conventional solder resists contain a photopolymerization initiator such as benzyl, benzoin ether, Michler's ketone, anthraquinone, acridine, phenazine, benzophenone, or α-acetophenone-based initiator, which does not have a sufficiently high photopolymerization potential only with a single wavelength light for example at 355 nm or 405 nm. Thus, photopolymerizable compositions containing a conventional photopolymerization initiator are extremely limited in their application range.
For this reason, photopolymerization initiators having a high photopolymerization potential even with a single wavelength light at 355 nm or 405 nm and compositions prepared by using the photopolymerization initiator were proposed (see e.g., Patent Documents 1 and 2). However, although these methods have a sufficient photopolymerization potential even with a single wavelength light at 355 nm or 405 nm, they still have problems, for example, that it was not possible to obtain simultaneously high in-depth hardenability and surface hardenability because of its very high photopolymerization rate and that there is a drastic drop in sensitivity because of inactivation of the photopolymerization initiator on the circuit after heat treatment and also exfoliation of the film on the copper circuit.
In addition, direct imaging by using a laser beam, which is generally performed under an atmospheric environment, often is sensitive to reaction inhibition by oxygen, and thus, removal of the surface of some desired region occurs in the developing process of removing an undesired region after exposure, consequently causing a problem of low glossiness. Further, low glossiness of the solder resist, which is used for protection of printed wiring boards, causes problems of unfavorable appearance and also low chemical resistance as well as low electrical properties of the part of the surface where the hardening reaction is insufficient.
Photocurable resins improved in photohardening efficiency by increase in the density of photosensitive groups were proposed (e.g., Patent Document 3), but they are still not compatible with laser direct imaging.
Patent Document 1: Jpn. Pat. Appln. KOKAI Publication No. 2001-235858 (claims)
Patent Document 2: WO02/096969 (claims)
Patent Document 3: Jpn. Pat. Appln. KOKAI Publication No. 9-80749 (claims)